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Old 03-24-2011, 06:28 PM   #1
Registered: May 2003
Location: Chicago
Distribution: Slackware 10
Posts: 45

Rep: Reputation: 15
Linux router MTU between 2 interfaces

Gotta question. Have a linux rotuter with 2 physical interfaces and a ipip tunnel. The one interface is connected to my isp, and the other is connected to my LAN. Routing is set up over the ipip tunnel to my office so I can route information between my office and my home.

The tunnel has an MTU of 1480 and connects via eth0, eth1 has an mtu of 1500.. Everything communicates fine, except when a packet is transfered that has a greater mtu than 1480 (unless I set all my machines at my house to have an mtu of 1480).. Any way to set the routers so they auto-fragment the packets if above 1480 and reconstruct them on the other side of the tunnel?

Thanks all
Old 03-25-2011, 05:51 AM   #2
Registered: Mar 2009
Posts: 249

Rep: Reputation: 27
You could do that by increasing the tunnel MTU and and setting the nopmtudisc flag on the tunnel, but that is a bad solution to the problem.

What should happen if the packet is too large is that router should fragment the packet if the don't fragment flag is not set, and the end host should reassemble it. If the don't fragment flag is set, the router should send an ICMP error packet to the sender which should retry with a smaller packet.

I suggest you look for firewalls blocking ICMP.


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